1. Field of the Invention
This invention relates to an organic EL device which is useful in display applications, and to a method of manufacturing such an organic EL device. More specifically, this invention relates to an organic EL device that prevents intrusion of water from the outside environment and exhibits excellent light emission efficiency over long periods of time, and to a method of manufacturing such an organic EL device.
2. Description of the Related Art
In display applications in recent years, there has been active research on organic EL devices employing light-emitting organic EL elements. Organic EL devices are expected to achieve high light emission brightness and light emission efficiency. This is because high current densities can be achieved at low voltages. In particular, high hopes are being placed in the display engineering field on high-definition multicolor light-emitting organic EL devices capable of multicolor display and in particular full-color display.
In addition to the realization of high-definition display, the attainment of long-term stability, including color reproducibility, is an important problem for the commercialization of organic EL displays as color display devices. However, multicolor light-emitting organic EL displays have the drawback of suffering marked degradation of light emission characteristics (current-brightness characteristics) as a result of driving over a fixed period.
The growth of dark spots are a representative cause of this decline in light emission characteristics. A “dark spot” is a light emission point defect. Such dark spots are thought to occur due to progressive oxidation or agglomeration of material in a layer comprised by an organic EL element during driving or during storage, due to oxygen or water within the element. Dark spot growth occurs not only when current is passed, but during storage as well. In particular, dark spot growth is thought to (1) be accelerated by oxygen or water existing in the outer environment surrounding the element, (2) be affected by oxygen or water exiting as adsorbed material in a constituent layer, and (3) be affected by water adsorbed by components used in device manufacture, or by the intrusion of water at the time of manufacture. If this growth continues, dark spots spread over the entire light-emitting face of the organic EL device.
As means of the prior art to prevent intrusion of water into constituent layers of organic EL elements, methods in which the organic EL element is sealed using metal housings or glass plates, or methods in which a desiccant is arranged within the space in which the organic EL element is sealed, have been employed. However, in order to take advantage of the light weight and thin shape which are features of organic EL devices, technology for sealing using thin films without employing desiccants has attracted attention.
As thin films for sealing, silicon nitride, silicon oxynitride and the like are used. However, in order to suppress damage to the light-emitting layer during film deposition of these materials, temperature increases at the film deposition surface must at least be suppressed to the glass transition temperature of the light-emitting layer or below. For this reason, film deposition methods which have been developed as part of semiconductor processes cannot be applied to organic EL devices, and there is the problem that a thin film for sealing, having adequate moisture exclusion properties, cannot be formed.
On the other hand, in Japanese Patent Application Laid-open No. 2005-209356, a silicon oxynitride film formed by a sputtering method or a plasma CVD method is proposed as a thin film for sealing that can be applied to organic EL devices (see Japanese Patent Application Laid-open No. 2005-209356). In Japanese Patent Application Laid-open No. 2005-209356, in order to achieve both high gas barrier properties and high optical transmissivity, the use of a gradient silicon oxynitride film the composition of which is changed continuously and in a gradient, and the use of a two-layer layered film of a silicon oxynitride film and a silicon nitride film, are disclosed.
And, in Japanese Patent Application Laid-open No. 2005-222778, a layered film of a silicon nitride film having compressive stress and a silicon nitride film having tensile stress is proposed as a thin film for sealing that can be applied to organic EL devices (see Japanese Patent Application Laid-open No. 2005-222778). In Japanese Patent Application Laid-open No. 2005-222778, the ability to control the magnitudes of the compressive stress and tensile stress, and the ability to control the refractive index of silicon nitride film by means of the number of Si—H bonds formed in silicon nitride film are disclosed. However, because the main bonds in silicon nitride films formed in the layered film are Si—N bonds, the change in lattice constants between the layers is small, defects in the layer serving as the substrate are carried over, and so there is the problem that silicon nitride film containing defects is formed.
In recent years, in order to improve the aperture ratio of organic EL devices employing active matrix driving, so-called top-emission structure devices, in which light is emitted from the opposite side of the substrate on which switching circuitry comprising TFTs and the like is fabricated, have become the mainstream. In this structure, transparent electrodes and sealing film are formed on the organic EL layer, and light emitted from the organic EL layer passes through the sealing film and is emitted to the outside. The refractive indices of the ITO and IZO used in the upper electrodes are approximately 2, and there is a large difference with the refractive indices of the adhesion layer and protective substrate formed in the direction of the outside air and direction of light emission. In Japanese Patent Application Laid-open No. 2005-209356, the refractive index of the sealing film in the direction of light propagation is not stipulated, and so there is the drawback that transmissivity is low.
Further, when using a method in which the proportion of film deposition gas is changed continuously during the process of deposition of the protective film, because of the continuous growth of interfaces and other film defects formed due to differences in the film growth direction arising from misalignment of the atomic order, step formation and the like, such film defects may cause local declines in moisture exclusion properties.
Accordingly, an object of this invention is to provide an organic EL device having long-term stability through the use of a protective layer having high visible-light transmissivity and excellent moisture exclusion properties. A further object of this invention is to provide a method of manufacturing the above-described organic EL device.